Bluetooth Serial Port Profile: Latency requirement

[vinayc1008]

I have a device having serial port interface. I am planning to replace the interface by Bluetooth Serial Port Profile. But i have a specific reqirement in that system that, the data from the device has to reach PC within 50ms. Is is possible to achieve this latency requirement using Bluetooth Serial port Profile.

[connekt]

Originally Posted by vinayc1008

I have a device having serial port interface. I am planning to replace the interface by Bluetooth Serial Port Profile. But i have a specific reqirement in that system that, the data from the device has to reach PC within 50ms. Is is possible to achieve this latency requirement using Bluetooth Serial port Profile.

Yes i think you should be fine
the latency introduced for each bluetooth packet is about 25 to 40 ms.
What kind of data are you sending? do you have lots of data to send at a time or is it small payloads that are spread apart?

[vinayc1008]

data lenght is of 10 bytes and there can be maximum of one such data packet every 50ms.
Is there any way where i can know the latency introduced by each layer in SPP.
ie latency in Baseband, LMP, L2CAP, RFCOMM and SDP

[vinayc1008]

Originally Posted by connekt

Yes i think you should be fine
the latency introduced for each bluetooth packet is about 25 to 40 ms.
What kind of data are you sending? do you have lots of data to send at a time or is it small payloads that are spread apart?

data lenght is of 10 bytes and there can be maximum of one such data packet every 50ms.
Is there any way where i can know the latency introduced by each layer in SPP.
ie latency in Baseband, LMP, L2CAP, RFCOMM and SDP

[connekt]

well you'll definitely be taking a hit on speed since you have such small payloads. That means there will be a latency added to every 10 bytes of data. If you had larger payloads, then the latency wouldn't affect the data transfer speed as much since you'd have a better ratio between the data transferred and the initial latency.

I'm not sure how you could find out about the latency introduced by each layer. maybe you would be better off not using SPP and using HCI instead. Since it is a much lower level profile and therefore i would assume it would require less overhead for each packet.

[vinayc1008]

Originally Posted by connekt

well you'll definitely be taking a hit on speed since you have such small payloads. That means there will be a latency added to every 10 bytes of data. If you had larger payloads, then the latency wouldn't affect the data transfer speed as much since you'd have a better ratio between the data transferred and the initial latency.

I'm not sure how you could find out about the latency introduced by each layer. maybe you would be better off not using SPP and using HCI instead. Since it is a much lower level profile and therefore i would assume it would require less overhead for each packet.

Thanks, I will explore more on HCI.

And also i would like to explain more about the device.
The device has all the features of a keyboard and mouse integrated in it.
But in addition to that it has a buzzer which can be controlled by PC.
So i think the device cannot belong to Human Interface Device Standard, because HID doesnot give any command to control Buzzer in the device.
So I thought of going for SPP.

Meanwhile, I have gone through the specification of HID profile.
There is a latency worksheet provided for the HID (Bluetooth Specification, HID Profile Section 6.6.4). According to that specification, the maximum latency introduced by the HID profile stack is around 28.76ms. Since the HID profile has more layer than SPP, can I assume that delay in SPP is lesser that HID?

[connekt]

hmmm i'm not sure but I would guess that it's not a great assumption. I'm not familiar with HID but I'm guessing that HID and SPP have different layers and build on top of different stacks. I would assume that different stacks have different latencies depending on complexity of each packet.

But I am not an expert, my main source of experience comes from developing a product that I posted on this forum
http://www.wifi-forum.com/wf/showthread.php?t=129971

It uses the Serial Port Profile and I did some research on the latency introduced in order to understand the speeds I was witnessing.